Optical exposure apparatus and photo-cleaning method

ABSTRACT

Optical exposure apparatus and methods of using same, for patterning a workpiece and photo-cleaning the optical components in the apparatus, which can be contaminated by moisture and organic compounds in the atmosphere. The apparatus comprises an illumination optical system having a light source and one or more optical components, and a projection lens having an object plane and an image plane and one or more optical components. The optical exposure apparatus includes an exposure optical path or an exposure light beam through a predetermined space in the optical exposure system. An optical path deflection member for deflecting light is introduced into the exposure optical path so as to create a second optical path that differs from the exposure optical path. Also disclosed is a method of photo-cleaning the aforementioned optical components, including the steps of forming an exposure optical path and then changing this path to create a second optical path that differs from the exposure optical path. An additional method of photo-cleaning a projection lens is disclosed, including the steps of providing an illumination light beam along an optical axis, providing and inserting a photo-cleaning optical member having refractive power into the beam, and refracting the beam so as to illuminate the lens surfaces comprising the projection lens.

This is a continuation-in-part of application Ser. No. 09/064,335, filedApr. 22, 1998.

FIELD OF THE INVENTION

The present invention pertains to optical exposure apparatus andmethods, and in particular to such apparatus and methods wherein theoptical materials comprising the apparatus are susceptible to changes intheir optical properties due to the presence of matter, such as moistureand organic compounds, in the atmosphere surrounding the apparatus.

BACKGROUND OF THE INVENTION

Present-day manufacturing of semiconductor devices, which includesintegrated circuits, liquid crystal displays, thin film magnetic heads,and the like, employs optical exposure apparatus and methods. Theincreasing degree of integration of such semiconductor devices hasplaced increasing demands on the optical exposure apparatus to achievehigher levels of resolution. The resolution of an optical exposureapparatus can be approximated by the relation

R=k×λ/NA _(P)

wherein, R is the “resolution” or resolving power of the opticalexposure apparatus (i.e., the size of the smallest feature that can beprinted), λ is the wavelength of the exposure light, NA_(P) is thenumerical aperture of the projection lens included therein, and k is aconstant which depends upon on the type of recording medium used and theprocess for developing the images formed therein.

To keep up with the increasing degree of integration of semiconductordevices, continuing efforts are being made, as can be understood fromthe above formula, to increase the resolution of the optical exposureapparatus by shortening the wavelength of the exposure light and/orincreasing NA_(P). In recent years, KrF (krypton fluoride) excimerlasers having an output wavelength of 248 nm have been used for theexposure light source. Moreover, projection lenses NA_(P) of 0.6 orgreater have been commercialized, and features as small as 0.25 micronshave been realized.

More recently, in an effort to increase resolution ArF (argon fluoride)excimer lasers having an output wavelength of 193 nm have been gainingattention as a successor light source to KrF excimer lasers. Thisreduction in wavelength could, in principle, allow the printing offeatures 0.18 microns or less. However, optical exposure apparatusoperable at deep ultraviolet (“DUV”, i.e., less than 200 nm) wavelengthsare difficult to realize. One reason for this is that in this wavelengthregion, the materials available for the necessary optical components arecurrently limited to quartz and calcium fluoride (fluorite). For thesematerials to be suitable for use in DUV optical exposure apparatuses,they must have sufficient transmittance and internal uniformity (aninternal transmittance of 0.995/cm or greater has been achieved withfused quartz, and negligible levels of absorption have been achievedwith calcium fluorite). Also, optical components made from thesematerials require an anti-reflection coating on their surfaces when usedat DUV wavelengths, to increase light transmission.

However, even with anti-reflection coatings and minimum levels ofabsorption, the optical characteristics of fused quartz and calciumfluorite can change due to the heat generated by surface contaminants,which absorb DUV light. Also, moisture and organic substances in the aireasily adhere to the lens surfaces of lenses used in the opticalexposure apparatuses discussed above. The lens surfaces can becontaminated by these contaminants during the manufacturing of theoptical exposure apparatus, as well as during its maintenance. Inparticular, since these contaminants strongly absorb light having awavelength of less than 200 nm, transmittance in optical exposureapparatus that use exposure light of less than 200 nm is reduced due tosuch contaminants adhering to the lens surfaces. For instance, it hasbeen discovered that the transmittance of optical components made fromfused quartz and calcium fluoride drops rapidly when exposed to moistureor organic compounds. The amount of this absorption, which can reach upto 0.01 per lens surface, is large compared to the absorption due to thematerial itself or the surface anti-reflection coatings. Therefore, itis necessary to keep the surfaces of fused quartz or calcium fluoriteoptical components free of such contaminants.

Japanese patent application Kokai No. Hei 7-294705 discloses a techniquerelating to a method for photo-cleaning individual optical componentswith light (hereinafter, “photocleaning”). In the photo-cleaningtechnique disclosed therein, the contaminants adhering to the lenssurfaces, in the manner described above, separate from the lens surfaceswhen irradiated with ultraviolet light, effectively cleaning the lenssurfaces. Further, when exposure light of less than 200 nm used in theoptical exposure apparatus is ultraviolet light, the contaminantsadhering to the lens surfaces may be photo-cleaned by operating theoptical exposure apparatus and irradiating the lenses of the opticalsystem with the exposure light. However, this technique does notdisclose a method for photo-cleaning all, or the essential optical partsof, the optical components of an optical exposure apparatus after theapparatus has been assembled. It has been discovered by the presentinventors that the temporary photo-cleaning of individual opticalcomponents by exposing them to DUV light actually facilitates the laterabsorption of ambient moisture and organic compounds onto the surfacesof the optical components. Consequently, even if individual opticalcomponents are photo-cleaned using DUV light, it is extremely difficultto assemble the optical components to form a projection exposure system,and then isolate those components completely from moisture, organiccompounds, and other contaminants. This has been a major impediment inrealizing a robust DUV projection exposure system.

Nevertheless, the numerical aperture NA_(I) of illumination opticalsystems used in projection exposure apparatuses are generally smallerthan NA_(P) of projection lens. Consequently, only one part of the NAregion of a projection lens (i.e., the region corresponding to NA_(I))is illuminated if the illumination light from the illumination opticalsystem impinges directly onto the projection lens. The result is thatonly the illuminated region is photo-cleaned. This is problematicbecause cleaned regions having high transmittance andcontaminant-adhered regions having low transmittance arise on the lenssurfaces, resulting in unevenness (i.e., non-uniformity) in the amountof light in the mask pattern image. This unevenness is due totransmittance unevenness and degradation of the resolving power causedby a reduction in the effective NA of the projection lens, which causesa drop in the imaging performance of the projection exposure apparatus.This has been another major impediment in realizing a robust DUVprojection exposure system.

SUMMARY OF THE INVENTION

The present invention pertains to optical exposure apparatus andmethods, and in particular to such apparatus and methods wherein theoptical materials comprising the apparatus are susceptible to changes intheir optical properties due to the presence of organic matter, such asmoisture and organic compounds, in the atmosphere surrounding theapparatus.

More particularly, a first aspect of the invention is an opticalexposure apparatus for forming an image on a photosensitive substrate.The apparatus comprises an illumination optical system having a lightsource and a light beam. Next to the illumination optical system is areticle having a pattern. Next to the reticle on the opposite side ofthe illumination optical system is a projection lens. The projectionlens and the illumination optical system have a predetermined spacetherein. An exposure optical path (i.e., the light path associated withperforming an exposure) passes through this predetermined space. Anoptical path deflection member is removably arranged in thepredetermined space so as to cause a deflection in the exposure opticalpath to form a second optical path. The second optical path differs fromthe exposure optical path. The second optical path is the light pathassociated with performing photo-cleaning. The optical path deflectionmember can be, for example, a rotating prism or one or more movingoptical components (including lenses and mirrors).

In a second aspect of the invention, instead of an optical pathdeflection member, a light diffusing member is placed in the light beamassociated with performing an exposure (i.e., the exposure light beam)so as to create a second light beam that is larger than the exposurelight beam. This second light beam is the light beam associated withperforming photo-cleaning.

In a third aspect of the invention, the optical path deflection memberor light diffusing member is removably disposed within the predeterminedspace so that photo-cleaning can be performed between exposures.

A fourth aspect of the invention is a method of photo-cleaning anoptical exposure apparatus for forming an image on a photosensitivesubstrate. The method comprises the steps of first, forming an exposureoptical path in a predetermined space within an illumination opticalsystem and a projection lens. The second step is preventing thephotosensitive substrate from being in said exposure optical path. Thethird step is changing the exposure optical path to an optical path thatdiffers from said exposure optical path.

A fifth aspect of the invention is a method of photo-cleaning aprojection lens having an object plane. The method comprises a firststep of providing an illumination light beam along an optical axis. Thesecond step is providing and inserting a photo-cleaning optical memberhaving refractive power into the illumination light beam. The third andlast step is refracting the illumination light beam with thephoto-cleaning optical member so as to illuminate lens surfaces of oneor more lenses included in the projection lens.

A sixth aspect of the invention is an optical exposure apparatus capableof imaging a pattern present on a mask onto a photosensitive substrate.The apparatus comprises an illumination optical system with a lightsource for generating a light beam. A projection lens having an objectplane is disposed adjacent the illumination optical system. Aphoto-cleaning optical member is removably provided between theillumination optical system and the projection lens and within the lightbeam so as to refract the light beam to impinge of the projection lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a deflection prismand a single light detector located on the workpiece stage;

FIG. 2 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a deflection prism, abeam splitter for creating a second optical path, and a single lightdetector located therein;

FIG. 3 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a deflection prism, alight detector located on the workpiece stage, a beam splitter forcreating a second optical path, and a light detector located therein;

FIG. 4 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a deflection prism, abeam splitter for creating a second and third optical paths, and a lightdetector located in each of the second and third optical paths;

FIG. 5 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a rotating mirror;

FIG. 6 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a movable lenscomponent;

FIG. 7 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a diffusion plate,and a single light detector located on the workpiece stage;

FIG. 8 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a diffusion plate, abeam splitter for creating a second optical path, and a single lightdetector located therein;

FIG. 9 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a diffusion plate, alight detector located on the workpiece stage, a beam splitter forcreating a second optical path, and a light detector located therein;

FIG. 10 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a diffusion plate, abeam splitter for creating a second and third optical paths, and a lightdetector located in each of the second and third optical paths;

FIG. 11 is a schematic diagram of an embodiment of the optical exposureapparatus of the present invention, which includes a housing, a variableaperture stop, and gas supply and gas discharge systems connected to thehousing and the projection lens;

FIG. 12 is a front view of the variable aperture stop utilized in theoptical exposure apparatus of FIG. 11, wherein the aperture stop haseight different apertures;

FIG. 13 shows the superposition of several of the apertures of theaperture stop of FIG. 12;

FIG. 14 is a schematic diagram of a preferred embodiment of a projectionlens of the present invention which has several lens chambers and supplyand discharge conduits for permitting gas flow through the chambers;

FIG. 15 is a schematic diagram of a preferred embodiment of aphoto-cleaning apparatus of the present invention;

FIG. 16 is a schematic diagram of an optical exposure apparatusaccording to the present invention, which includes a concave lens and aconvex lens as a photo-cleaning optical member;

FIG. 17 is the optical exposure apparatus of FIG. 16 with theillumination optical apparatus shown in greater detail;

FIG. 18 illustrates the path of the exposure light beams from theillumination optical apparatus before and after passing through a maskarranged at the object plane;

FIG. 19 illustrates the path of the exposure light beams from theillumination optical apparatus before and after passing through a maskarranged at the object plane, and then passing through the projectionlens to the workpiece;

FIG. 20 illustrates the path of the exposure light beams from theillumination optical apparatus before and after passing through aphoto-cleaning optical member comprising a diffusion plate;

FIG. 21 illustrates the path of the exposure light beams from theillumination optical apparatus before and after passing through aphoto-cleaning optical member comprising a negative lens having negativerefractive power;

FIG. 22 illustrates the path of the exposure light beams from theillumination optical apparatus before and after passing through aphoto-cleaning optical member comprising a positive lens having positiverefractive power;

FIGS. 23a and 23 b are schematic diagrams indicating the focal length ffor a photo-cleaning optical member comprising a negative lens and apositive lens, respectively;

FIGS. 24a and 24 b illustrate the path of the exposure light beams fromthe illumination optical apparatus before and after passing through aphoto-cleaning optical member comprising a negative lens (FIG. 24a) anda positive lens (FIG. 24b), and then through the projection lens;

FIG. 25 is a graph representing a time-varying characteristic of thetransmittance of a projection lens;

FIG. 26 is a flowchart of the overall exposure and photo-cleaningprocedure of the present invention; and

FIG. 27 is a flow chart of the specific photo-cleaning procedure of Step2 of the flow chart of FIG. 26.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to optical exposure apparatuses andmethods, and in particular to such apparatuses and methods wherein theoptical materials comprising the apparatus are susceptible to changes intheir optical properties due to the presence of matter, such as moistureand organic compounds, in the atmosphere surrounding the apparatus. Suchapparatuses include, for example, DUV-wavelength optical exposure orprojection exposure apparatuses for semiconductor manufacturing, whichutilize optical components made from synthetic quartz and/or calciumfluoride.

Referring to FIG. 1, optical exposure apparatus 5 includes, along anoptical axis A, a light source 10 which emits a light beam 11. Lightsource 10 may be, for example, an ArF excimer laser emitting light at193 nm. Adjacent light source 10 is a beam-shaping optical system 12,which adjusts the shape of light beam 11. An optical integrator 13 islocated immediately adjacent optical system 12. Optical integrator 13has a plurality of lens segments (three are shown) and can be, forexample, a fly's-eye lens (i.e., a bundle of elongated optical elements,similar to that shown in FIG. 1), or an internal-reflection light pipe.Further, optical integrator 13 is not limited to being a single opticalcomponent, but may be a plurality of optical integrators arranged inseries to achieve greater light beam uniformity.

Further included in optical exposure apparatus 5 adjacent opticalintegrator 13 is a first image plane P1 at which an aperture stop 14 islocated. Adjacent aperture stop 14 is a front condensing lens 15 a and arear condensing lens 15 b. Adjacent rear condensing lens 15 b is asecond image plane P2 at which an adjustable field stop 16 is located.Adjacent field stop 16 and second image plane P2 is a first objectivelens 17, a fold mirror 18 and a second objective lens 19. Theabove-described optical components from front condensing lens 15 athrough second objective lens 19 constitute a condenser optical system.The above-described optical components from light source 10 to secondobjective lens 19 constitute an illumination optical system.

With continuing reference to FIG. 1, a reticle 20 is located opticallyconjugate to field stop 16, at the illumination optical system imageplane (not shown) as defined by objective lenses 17 and 19. Reticle 20contains small patterns (not shown) to be imaged. Adjacent reticle 20 isa projection lens 21 having an object plane OP and an image plane IP.Object plane OP of projection lens 21 is located optically conjugate tofield stop 16, as defined by objective lens 17 and 19 (thus, reticle 20is located at object plane OP). Projection lens 21 may be, for example,a dioptric or catadioptric high-performance DUV photolithographic lenscomprising fused quartz and/or calcium fluoride optical components.Disposed at or near image plane IP of projection lens 21 is a workpiece22 having a surface 22 s. Workpiece 22 may be, for example, a wafercoated with a photosensitive recording medium, such as photoresist.Workpiece 22 is positioned and held in place by a movable workpiecestage 23. A light detector 24 is located on workpiece stage 23, and canbe moved into the optical path underneath projection lens 21 to measurethe intensity of the light passing therethrough. Light detector 24 isconnected to a light detection system 26, which is connected to acontrol system 27. Here, light detection system 26 has the function fordetecting light transmittance. Light detector 24 may be, for example, aphoto-electric sensor, which converts light energy into an electricalsignal.

With continuing reference to FIG. 1, optical exposure apparatus 5further includes a rotatable deflection prism 30. Deflection prism 30 isinserted and removed from a predetermined space within optical exposureapparatus 5, which contains an optical path (i.e., light rays R) betweensecond objective lens 19 and reticle 20, by a drive system 32 which isconnected to control system 27. Also included in optical exposureapparatus 5 is a shutter 34. Shutter 34 is inserted and removed from theoptical path (i.e., light rays R) between projection lens 21 andworkpiece 22 by a drive system 36, which is also connected to controlunit 27. When shutter 34 is inserted in the optical, the shutter is“closed,” and when the shutter is removed from the optical, it is“open.”

With continuing reference to FIG. 1, the operation of optical exposureapparatus 5 in exposing workpiece 22, in the absence of deflection prism30 and shutter 34, is now described. First, light source 10, whenactivated, emanates light beam 11, which passes through beam-shapingoptical system 12. Beam-shaping optical system 12 adjusts the shape(e.g., collimates and spatially filters) light beam 11 so that it hasthe appropriate dimensions prior to the light beam entering opticalintegrator 13. Optical integrator 13 then forms a plurality of images orsecondary light sources SLS at image plane P1. Aperture stop 14, locatedat image plane P1, serves to limit the extent of light rays R emanatingfrom secondary light sources SLS. Front and rear condensing lenses 15 aand 15 b then superimpose each light source of secondary light sourcesSLS to form a single, overlapping image (not shown) at image plane P2,as indicated by the path of light rays R. The image formed at imageplane P2 has an intensity distribution that is highly uniform due to thesuperposition of the light sources of secondary light sources SLS.

Field stop 16, located at second image plane P2, serves to define theshape of the image formed at image plane P2. Light from the image formedat image plane P2 is relayed by first objective lens 17, fold mirror 18and second objective lens 19 to object plane OP of projection lens 21(coincident with the illumination optical system image plane, notshown), at or near which reticle 20 resides. Reticle 20 is thusuniformly illuminated, and an image of the pattern located thereon isprojected onto surface 22 a of workpiece 22 by projection lens 21. Theoptical path defined by light rays R (with or without reticle 20present) is referred to herein as the exposure optical path. This is theoptical path light from light source 10 follows through thepredetermined space within the optical exposure apparatus when theoptical exposure apparatus is arranged to perform an exposure to patternthe workpiece.

As mentioned above, for DUV optical exposure apparatus using fusedquartz and/or calcium fluoride optical components, moisture and organicmatter in the atmosphere surrounding the optical exposure apparatustends to adhere to and contaminate the surfaces of the opticalcomponents. This changes the optical properties of the components, inparticular reducing the transmittance of the refractive opticalcomponents and the reflectance of reflecting optical components. Whenlight source 10 is a DUV light source and is activated (oralternatively, when aperture stop 14 or field stop 16 is opened) toperform an exposure of workpiece 22, the contaminated optical componentsare, to a certain extent, photo-cleaned by the DUV light passingtherethrough. However, the present inventors have found thatrecontamination can quickly occur between exposures. Also, thephoto-cleaning that does occur during exposure of the workpiece reducesthe intensity of the exposure beam.

Accordingly, in the present invention, optical exposure apparatus 5 isconfigured so that, prior to exposing workpiece 22, a photo-cleaningprocess can be performed whereby light from light source 10 passesthrough each optical component in a manner that eliminates contaminationfrom their respective surfaces. In this regard, the role of deflectionprism 30 and shutter 34 is to provide light of sufficient intensity forphoto-cleaning over the entire area desired for one or more opticalcomponents in optical exposure apparatus 5. Deflection prism 30 deflectslight in the exposure optical (i.e., light rays R) entering projectionlens 21, by a deflection angle Δ (not shown), which is a function of theprism wedge-angle α (not shown). Deflection prism 30 is rotatable bydrive system 32 such that light rays R can be deflected to pass throughportions, including the peripheral portions, of the optical componentsin projection lens 21 (i.e., the portions of the optical componentsother than those traversed by the exposure optical path).

To appreciate the importance of deflection prism 30 in performingphoto-cleaning, it is useful to consider the photo-cleaning optionsavailable in optical exposure apparatus 5 of FIG. 1 without it. Thefirst option is to perform photo-cleaning with reticle 20 removed. Thiswould be preferred because the patterns on the reticle absorb light,reducing the intensity of the light passing through projection lens 21resulting in less efficient photo-cleaning. However, the numericalaperture NA_(P) of projection lens 21 is generally greater thannumerical aperture NA_(I) of the illumination optical system so that theprojection lens can collect light diffracted from the pattern on reticle20. This diffracted light is part of the exposure optical path andtraverses certain peripheral portions of the optical components ofprojection lens 21. Accordingly, with reticle 20 removed, there is morelight available for photo-cleaning, but the light will only traverse theoptical components of projection lens 21 out to a radial distancedefined by NA_(I). The other option is to keep reticle 20 in placeduring photo-cleaning, so that certain peripheral portions of theoptical components receive diffracted light. However, the diffractedlight from typical reticle patterns does not have the necessaryintensity for effective photo-cleaning.

In performing photo-cleaning using deflection prism 30, it is desirableto arrange deflection prism 30 such that deflection angle Δ satisfiesthe relationship Δ≧NA_(P)−NA_(I). This ensures that the peripheralportions of the optical components (including those portions traversedand not traversed by the exposure optical path) will be photo-cleaned.Note that there is no need to reduce NA_(I) during photo-cleaning, andit is generally preferable to set NA_(I) at its maximum value.

With continuing reference to FIG. 1, insertion of shutter 34 into theoptical path between projection lens 21 and workpiece 22 preventsexposure of workpiece 22 during photo-cleaning. Thus, replacement and/oraligning of the workpiece can be performed simultaneous withphoto-cleaning. In a preferred embodiment, shutter 34 has a surface 34 sfacing projection lens 21 that is reflective. Thus, light rays Rincident shutter surface 34 s are reflected back through projection lens21 and through the rest of optical exposure apparatus 5, therebyenhancing the photo-cleaning effect. Shutter 34 can also be tilted withrespect to optical axis A passing therethrough, by means of drive system36, thereby allowing for greater deflection of the light in the opticalpath, which increases the effectiveness of photo-cleaning.

The photo-cleaning process, as described above, can be performed with orwithout reticle 20 being present. However, when reticle 20 is presentthe time required for photo-cleaning increases due to light beingabsorbed by the reticle pattern. On the other hand, if reticle 20 isremoved, the time required for photo-cleaning is shorter, but there isthe added time is required for removing and inserting reticle 20.Accordingly, if the time needed for removing and inserting reticle 20makes the overall photo-cleaning process longer, then it may bepreferable to perform photo-cleaning with reticle 20 present.

Also, though deflection prism 30 is shown in FIG. 1 disposed betweenobjective lens 19 and reticle 20, deflection prism 30 can be disposedanywhere in optical exposure apparatus 5. For example, it may bedisposed between reticle 20 and projection lens 21, between aperturestop 14 and front condenser lens 15 a, between field stop 16 and reticle20, or within projection lens 21 itself. Generally speaking, deflectionprism 30 should be disposed “upstream” (i.e., on the light-source 10side) the optical component or components that require photo-cleaning.

With continuing reference to FIG. 1, light detector 24 located onworkpiece stage 23, is used to measure the intensity of light passingthrough projection lens 21. This is accomplished by moving workpiecestage 23 in a direction parallel to image plane IP such that lightdetector 24 is inserted into the optical exposure path. When shutter 34is opened, light is incident light detector 24, and the output (e.g., anelectrical signal) therefrom is inputted and stored in light detectionsystem 26. By comparing such light intensity measurements made atvarious points in time and stored in light detection system 26, thecombined transmittance of the optical components in optical exposureapparatus 5 can be immediately determined and compared to an “optimum”transmittance corresponding to the “cleanest” state of optical exposureapparatus 5. Accordingly, a decision for whether or not to start thephoto-cleaning process and whether or not to end the photo-cleaningprocess can be easily made. Alternatively, light detection system 26 caninclude an arbitrary threshold illumination intensity value, fordeciding when photo-cleaning should commence.

It may be desirable to measure the light intensity of optical exposureapparatus 5 of FIG. 1 at workpiece surface 22 s when shutter 34 isclosed. Thus, with reference now to FIG. 2, optical exposure apparatus40 represents another preferred embodiment of the present inventionwhich allows for this measurement. Optical exposure apparatus 40includes the same elements as optical exposure apparatus 5 of FIG. 1,except that light detector 24 is removed. Optical exposure apparatus 40further includes a beam splitter 42 disposed between aperture stop 14and front condensing lens 15 a, and a light detector 44 a. Beam splitter42 is disposed such that it creates an optical axis A′ at right anglesto optical axis A passing therethrough. Light detector 44 a is disposedalong optical axis A′.

In optical exposure apparatus 40, light reflected from reflectivesurface 34 s of shutter 34 when shutter 34 is closed, and travels backup through the optical exposure apparatus and backwards throughcondensing lens 15 a. A portion of this light is then reflected by beamsplitter 42 along optical axis A′ to light detector 44 a. In this way,the light transmittance of optical exposure apparatus 40 during thephoto-cleaning process can be measured in real-time, which is useful indeciding when the photo-cleaning process should terminate. Note thatthough beam splitter 42 is disposed between aperture stop 14 and frontcondensing lens 15 a, it can be disposed anywhere in optical exposureapparatus 40, including anywhere in the illumination optical system orprojection lens 21.

Certain optical exposure apparatuses include light sources whose lightbeam fluctuates in intensity. In such apparatuses, it is desirable to beable to distinguish between light beam intensity fluctuations due to thelight source and fluctuations in the transmittance of the opticalexposure apparatus due to contaminated optical components. Thus, withreference to FIG. 3, an optical exposure apparatus 50 represents apreferred embodiment of the present invention which can make such adistinction. Optical exposure apparatus 50 includes the same elements asoptical exposure apparatus 5 of FIG. 1, except that a beam splitter 42is added (as shown in optical exposure apparatus 40 of FIG. 2 anddescribed above), and light detector 44 b is disposed along axis A′ suchthat it receives light reflected by beam splitter 42 prior to the lightentering front condensing lens 15 a. The outputs of light detectors 24and 44 b are inputted into light detection system 26, where thetransmittance of optical exposure apparatus 50 is derived based on thedifference in the light detector outputs. In this configuration,fluctuations in the intensity of light emitted by light source 10 aredetected by both light detectors 24 and 44 b and can thus be subtractedfrom the transmittance calculation. Note again that beam splitter 42 maybe placed anywhere in optical exposure system 50. For example, if thetransmittance of projection lens 21 is to be determined, beam splitter42 can be disposed between reticle 20 and projection lens 21, withdetector 44 b disposed along optical axis A′ formed by beam splitter 42.

In optical exposure apparatus 50 of FIG. 3, the light intensity atworkpiece surface 22 s cannot be measured when shutter 34 is closed.Thus, with reference to FIG. 4, an optical exposure apparatus 55represents a preferred embodiment of the present invention, which is amodification of optical exposure apparatus 50 to allow for thismeasurement. Optical exposure apparatus 55 includes the same elements asoptical exposure apparatus of FIG. 2, and it further includes lightdetector 44 b, as shown in FIG. 3. In this configuration, light detector44 b detects light reflected by beam splitter 42 prior to the lightentering front condensing lens 15 a. Light detector 44 a detects lightreflected by reflective surface 34 s of shutter 34 which passes backthrough optical projection system 5 and backwards through frontcondenser lens 15 a. The outputs of light detectors 44 a and 44 b areinputted into light detection system 26. Comparison of the outputs fromlight detectors 44 a and 44 b allows for the transmittance of opticalexposure apparatuses between front condensing lens 15 a and shutter 34to be continuously monitored, regardless of the fluctuation in lightintensity emitted by light source 10, when shutter 34 is closed.

Optical exposure apparatuses 5, 40, 50, and 55, discussed above, allemploy deflection prism 30 to deflect the optical path duringphoto-cleaning. With reference now to FIG. 5, optical exposure apparatus60 represents a preferred embodiment of the present invention whichemploys a different means for deflecting the light path to performphoto-cleaning. Optical projection exposure apparatus 60 includes thesame elements as optical exposure apparatus of FIG. 1, except thatdeflection prism 30 is no longer present. Instead, fold-mirror 18 ismade movable in two-dimensions (FIG. 5 illustrates the movement inone-dimension). For example, fold-mirror 18 may be rapidly oscillated intwo dimension about its center or some other position. Moreover, themovement of fold-mirror 18 may be combined with a lateral shift. Themovement of fold-mirror 18, as described above, serves to deflect theoptical path to peripheral portions of the optical components in opticalapparatus 60 during photo-cleaning. Note that fold-mirror 18, likedeflection prism 30, can be located in at any other convenient locationin the illumination optical system.

With reference now to FIG. 6, optical exposure apparatus 65 representsanother preferred embodiment of the present invention which employs adifferent means for deflecting the light path to perform photo-cleaning.Optical exposure apparatus 65 includes the same elements as opticalexposure apparatus of FIG. 1, except that deflection prism 30 is nolonger present. Instead, objective lens 19 may be moved rapidly back andforth (i.e., oscillated) in a direction perpendicular to optical axis Apassing therethrough. Objective lens 19 may also simultaneously be movedalong optical axis A passing therethrough. The movement of objectivelens 19, as described above, serves to deflect light in the optical pathduring photo-cleaning. Note that other optical components in opticalexposure apparatus 65, such as condenser lenses 15 a and/or 15 b can becan be made movable in instead of, or in addition to, objective lens 19.

With reference now to FIG. 7, optical exposure system 70 is anotherpreferred embodiment of the present invention which employs yet anothermeans for deflecting light in the optical path to performphoto-cleaning. Optical exposure system 70 includes the same elements asFIG. 1, except that deflection prism 30 is absent. Instead, a lightdiffusing member, such as a diffusion plate 72 is inserted at or nearobject plane OP of projection lens 21. Diffusion plate 72 is connectedto drive system 32, which removes reticle 20 and inserts diffusion plate72 in its place for photo-cleaning. Drive system 32 also removesdiffusion plate 72 and inserts reticle 20 in its place for exposure.Diffusion plate 72 may be, for example, a fused quartz or calciumfluoride plate having a pattern etched in on one of its surfaces, suchas surface 72 s. The pattern on surface 72 s may be periodic, such adiffraction grating, to diffract light incident thereon, or may berandom to scatter light incident thereon. The degree to which diffusionplate 72 diffuses light can be characterized by a diffusion angle Δ′,similar to the deflection angle of the deflection prism, discussedabove. If the diffusion angle Δ′ is too large due to the pattern onsurface 72 s being too rough, it can be controlled by processing thesurface with chemicals such as hydrofluoric acid (hydrogen fluoride). Inaddition, if diffusion plate 72 includes a diffraction grating, thediffusion angle Δ′ can be controlled by adjusting the grating spacing.

As discussed above in connection with deflection prism 30 (see FIGS.1-4), it is generally preferred to set the diffusion angle Δ′ such thatΔ′=NA_(P)−NA_(I) to ensure the periphery of the optical componentsreceive light during photo-cleaning. Also, diffusion plate 72 can beused in analogous fashion to deflection prism 30 as it is used inoptical exposure apparatuses 5, 40, 50 and 55 shown in FIGS. 1-4. FIGS.7-10 show optical exposure apparatuses 70, 75, 80 and 85 which includediffusion plate 72 and which operate in the same manner as opticalexposure apparatuses 5, 40, 50 and 55, respectively, which includedeflection prism 30, as discussed above.

As can be appreciated from the above discussion, realizing a robust DUVoptical exposure apparatus and a method of patterning a workpiece usingthe apparatus is extremely difficult because of the aforementionedcontamination of the optical components. It is a particularly dauntingproblem because it is virtually impossible to reduce the humidity tozero or to eliminate the presence of organic compounds, which areubiquitous in a semiconductor manufacturing environment. Accordingly, itis essential to have an optical exposure apparatus and method thatincorporates photo-cleaning techniques which allow for a hightransmittance to be maintained when performing exposures.

Thus, with reference now to FIG. 11, there is shown a preferredembodiment of a projection exposure apparatus 100 of the presentinvention for patterning a workpiece. As shown in FIG. 11, laser lightis supplied as a nearly parallel light beam from light source 10, whichis, for example an ArF excimer laser that oscillates pulsed light havingan output wavelength of 193 nm. This laser light beam is guided to lighttransmitting window 104 on the side of projection exposure apparatus100. Accordingly, projection exposure apparatus 100 includes a chamber102 and filled with an inert gas like nitrogen, for example, to preventthe adherence of moisture and organic matter and the like in theatmosphere, to the optical elements and the like in chamber 104.

The laser light passing through light transmitting window 104 isreflected by reflective mirror 42, and guided to optical integrator(“fly's-eye lens”) 13. Fly's-eye lens 13 is constituted by bundlingnumerous lens elements, and a light source image is formed on the exitsurface side of these lens elements. Accordingly, at the exit surfaceside of fly's-eye lens 13, numerous light source images (i.e., secondarylight sources, not shown) corresponding to the number of lens elementscomprised thereof are formed.

Nevertheless, although one fly's-eye lens 13 is provided in the presentexample, a fly's-eye lens can also be provided as a second opticalintegrator between fly's-eye lens 13 and light source 10 or reflectivemirror 42. Furthermore, an internal reflection-type rod-shaped opticalmember can also be used in place of fly's-eye lens 13.

In addition, as discussed later in detail, a variable aperture stop 14(changing apparatus or changing means that can change the numericalaperture NA_(I) of the illumination optical system) which can set aplurality of aperture stops having a predetermined shape or apredetermined size at a position wherein the numerous secondary lightsources formed by the fly's-eye lens 13 are formed, is included.

The light beam from the numerous secondary light sources formed by thefly's-eye lens 13 is reflected by reflective mirror 18, and subsequentlycondensed by condenser optical system 19, comprising a plurality ofrefractive optical elements, like lenses. Consequently, the pattern,such as a circuit pattern, formed on reticle 20 is superimposingly anduniformly illuminated. Then, the image of the circuit pattern on reticle20 is formed on workpiece 22 (e.g., a wafer coated with resist) by meansof projection lens 21. Consequently, the resist coated on the wafer isexposed and the circuit pattern image is transferred onto workpiece 22.

Furthermore, projection lens 21 in the present example completelycomprises optical elements like refractive lenses, and an aperture stop110 is arranged at the position of the pupil (entrance pupil) ofprojection lens 21. In addition, aperture stop 110 and variable aperturestop 16 are at optically conjugate positions.

Accordingly, reticle 20 is supported by reticle stage 107 that movestwo-dimensionally within the plane orthogonal to the paper surface ofFIG. 11. The position information from measurement systems, like aninterferometer system (not shown), that measures the position of reticlestage 107 is input to control system 27 as the controlling means.Control system 27 controls the position of reticle stage 107 via a drivesystem (not shown) based on this position information.

In addition, workpiece 22 is mounted on workpiece stage 23 that movestwo-dimensionally in the plane orthogonal to the paper surface of FIG.11. The position information from measurement system 108, like aninterferometer system, that measures the position of this wafer stage 23is input to control system 27 as the controlling means. Control system27 controls the position of workpiece 23 via a drive system 109 based onthis position information.

In addition, in the projection exposure apparatus shown in FIG. 11, atransmittance measurement system is provided to perform measurementsrelated to the transmittance of the illumination optical system 42 to 19and projection lens 21. A first detector 44 b related to thetransmittance measurement system is arranged below reflective mirror 44b, and a second detector 24 related to the transmittance measurementsystem is arranged at one end of workpiece stage 23. Furthermore, athird detector 112 related to the transmittance measurement system isarranged at one end of reticle stage 107.

First, first detector 44 b provided below reflective mirror 42, (which,in this case, is partially transmitting) and detects the output of ArFexcimer laser light from light source 10 by photoelectrically detectingthe quantity of light, and the illumination intensity and the like ofthe transmitted light from one part of reflective mirror 42. Then, theoutput from first detector 44 b is input to control system 27 as thecontrolling means.

By setting, via drive system 109, second detector 24, provided at oneend of workpiece stage 23, in the plane wherein the workpiece 22 isarranged as the surface to be irradiated (or the image plane ofprojection lens 21), the quantity of light and illumination intensityand the like at the plane wherein wafer 22 is arranged (or the imageplane of projection lens 21) are detected. Then, the output from seconddetector 24 is input into control system 27 as the controlling means.

Furthermore, by setting, via a drive system (not shown), third detector112, provided at one end of reticle stage 107, in the plane whereinreticle 20 is preferably arranged, the quantity of light andillumination intensity and the like of the illumination light beamsupplied to the surface of reticle 20 are measured. Then, the outputfrom third detector 112 is input to control system 27 as the controllingmeans.

A divider unit and discrimination unit (not shown) are provided insidecontrol system 27. The divider unit outputs to the discrimination unitthe value of the output from second detector 24 divided by the outputfrom first detector 44 b (value corresponding to the light transmittanceof reflective mirror 42, condenser lens 19 and projection lens 21), orthe value of the output from third detector 112 divided by the outputfrom second detector 24 (value corresponding to the light transmittanceof projection lens 21). Then, the discrimination unit judges whether thetransmittance of the projection exposure apparatus has dropped bydiscriminating whether the output from the divider unit has reached apredetermined threshold value. Then, discrimination unit starts theexposure operation or continuously executes the exposure operation, orjudges whether to start the photo-cleaning operation.

In addition, a gas supply apparatus 120 for supplying an inert gas likenitrogen to chamber 102 and a plurality of spaces formed between aplurality of optical components inside projection lens 21, and a gasdischarge apparatus 130, for discharging the gas inside chamber 102 andthe gas inside a plurality of spaces formed between a plurality ofoptical components inside projection lens 21, are provided outside ofchamber 102. Furthermore, the inert gas is not limited to nitrogen, andgases like helium and argon can also be used.

Then gas supply apparatus 120 supplies inert gas (dried inert gas) likedried nitrogen to chamber 102 through pipe 122, and also supplies inertgas (dried inert gas) like dried nitrogen into projection lens 21through pipe 124. In addition, gas discharge apparatus 130 discharges,through pipe 132, the gas inside chamber 102, and also dischargesthrough pipe 134 the gas inside projection lens 21. The operation of gassupply apparatus 120 together with gas discharge apparatus 130 iscontrolled by control system 27.

Next, variable aperture stop 14 as the changing apparatus or changingmeans that changes the numerical aperture NA_(I) of the illuminationoptical system in projection exposure apparatus 100 discussed above,will be explained.

The σ value as the coherence factor (or, illumination coherence) isdefined by the formula σ=NA_(I)/NA_(P), wherein, NA_(I)=sin θ_(i) is thenumerical aperture of illumination optical system determined by rayR_(o) parallel to optical axis A passing therethrough from the outermostcircumference (outermost diameter) of variable aperture stop 14, andNA_(P)=sin θ_(o) is the numerical aperture on the illumination opticalsystem side of projection lens 21 determined by ray R_(o) parallel tooptical axis a passing through projection lens 21 from the outermostcircumference (outermost diameter) of aperture stop 110 in projectionlens 21, as shown in FIG. 11.

Aperture stop 110 is optically conjugate to variable aperture stop 14 inthe illumination optical system. Since the image of variable aperturestop 14 (image of secondary light sources) is formed at the pupil ofprojection lens 21, the σ value as the coherence factor can also bedefined by the following formula, wherein D₁₄ is the diameter of theimage of variable aperture stop 14, and D₁₁₀ is the diameter of aperturestop 110 of projection lens 21:

σ=D ₁₄ /D ₁₁₀.

Generally, projection exposure apparatus is constituted so that the avalue of the projection exposure apparatus in the photolithographyprocess is set to the range of 0.3-0.8. In the present example, variableaperture stop 14 shown in FIG. 11 is settably provided at the secondarylight source position formed by fly's-eye lens 13.

FIG. 12 shows a more concrete configuration of variable aperture stop 14shown in FIG. 11. As shown in FIG. 12, variable aperture stop 14 hasturret plate 140 having eight aperture stops 142 a-142 h formed on atransparent substrate, like quartz. Aperture stops 142 a, 142 e to 142 hhaving five circular apertures are for the purpose of actively changingthe σ value. Among these, three aperture stops 142 e, 142 f, 142 g areused during the actual exposure operation. The remaining two aperturestops 142 a, 142 h are used during the photo-cleaning operation.

Furthermore, the other three aperture stops having modified aperturesare for the purpose of improving the resolving power of projection lens21 by using them during the exposure operation. Among these, the twoaperture stops 142 c, 142 d have annular apertures with mutuallydiffering annular ratios. The remaining one aperture stop 142 b is astop having four off-centered apertures in order to form fouroff-centered secondary light sources.

Turret plate 140 having eight aperture stops 142 a-142 h is rotated viadrive system 146, such as a motor, shown in FIG. 11. One aperture stopamong the eight aperture stops, namely an aperture stop having a desiredaperture shape, is set at the secondary light source position by drivingsystem 146, which is controlled by control system 27.

FIG. 13 shows the images of aperture stops 142 a, 142 e to 142 h, havingcircular apertures of mutually differing sizes, formed on aperture stop110 inside projection lens 21. First, if aperture stop 142 e having thesmallest circular aperture is set in the illumination optical path,numerical aperture NA_(I) of the illumination optical system becomes thesmallest. At this time, the image of aperture stop 142 e having aperturediameter D_(140e) is formed inside projection lens 110 having aperturediameter D₁₁₀ to and the σ value is set to 0.4. In other words, therelation σ=D_(140e)/D₁₁₀=NA_(I)/NA_(P)=0.4 is established. Accordingly,if aperture stop 142 e is set inside the illumination optical path, thepattern of reticle 20 can be transferred onto workpiece 22 based on avalue of 0.4.

In addition, if aperture stop 142 f, having a circular aperture largerthan aperture stop 142 e, is set in the illumination optical path, thennumerical aperture NA_(I) of the illumination optical system becomeslarger than when aperture stop 142 e was set in the illumination opticalpath. At this point, the image of aperture stop 142 f having aperturediameter D_(140f) is formed inside projection lens 21 having aperturediameter D₁₁₀, and the σ value is set to 0.6. In other words, therelation σ=D_(140f)/D₁₁₀=NA_(I/NA) _(P)=0.6 is established. Accordingly,if aperture stop 142 f is set inside the illumination optical path, thepattern of reticle 20 can be transferred onto workpiece 22 based on a σvalue of 0.6.

In addition, if aperture stop 142 g, having a circular aperture largerthan aperture stop 142 f, is set in the illumination optical path, thennumerical aperture NA_(I) of the illumination optical system becomeslarger than when aperture stop 142 f was set in the illumination opticalpath. At this point, the image of aperture stop 142 g having aperturediameter D_(140g) is formed inside projection lens 21 having aperturediameter D₁₁₀, and the a value is set to 0.8. In other words, therelation σ=D_(140g)/D₁₁₀=NA_(I)/NA_(P)=0.8 is established. Accordingly,if aperture stop 142 g is set inside the illumination optical path, thepattern of reticle 20 can be transferred onto workpiece 22 based on a avalue of 0.8.

Furthermore, if aperture stop 142 h, having a circular aperture largerthan aperture stop 142 g, is set in the illumination optical path, thennumerical aperture NA_(I) of the illumination optical system becomeslarger than when aperture stop 142 g was set in the illumination opticalpath. At this point, the image of aperture stop 142 h, having aperturediameter D_(140h) the same size as the aperture diameter D₁₁₀ ofaperture stop 110, is formed, and the σ value is set to 1.0. In otherwords, the relation σ=D_(140h)/D₁₁₀=NA_(I)/NA_(P)=1.0 is established.Accordingly, if aperture stop 142 h is set in the illumination opticalpath, the illumination light beam is sufficiently guided as far as theeffective diameter of the optical elements like lenses comprisingcondenser lens 19 of the illumination optical system, the effectivediameter of optical elements like lenses comprising projection lens 21,and as far as the part beyond the effective diameters of these opticalelements. Consequently, moisture and organic matter and the likeadhering to the surfaces of these optical elements can be eliminated bythe photo-cleaning effect due to the exposure illumination light beam.

In addition, if aperture stop 142 a, having a circular aperture largerthan aperture stop 142 h, is set in the illumination optical path, thennumerical aperture NA_(I) of the illumination optical system becomeslarger than when aperture stop 142 h was set in the illumination opticalpath. At this point, the image of aperture stop 142 a having aperturediameter D_(140a) is formed so that it includes aperture stop 142 ahaving aperture diameter D₁₁₀, and the a value is set to 1.2. In otherwords, the relation σ=D_(140a)/D₁₁₀=NA_(I)/NA_(P)=1.2 is established.Accordingly, if aperture stop 142 a is set in the illumination opticalpath, the illumination light beam is sufficiently guided as far as theeffective diameter of the optical elements like lenses comprisingcondenser lens 19 of the illumination optical system, the effectivediameter of optical elements like lenses comprising projection lens 21,naturally, and as far as the lens circumference part beyond theeffective diameters of these optical elements. Consequently, an effectcan be sufficiently obtained wherein moisture and organic matter and thelike adhering to the surfaces of these optical elements isphoto-cleaned.

Next, the operation of the present example is explained. First, itbecame apparent through various experiments that, as shown in FIG. 11,the transmittance of the optical system part of projection exposureapparatus 100 drops if the illumination light beam as the exposure lightis not guided to the optical system part of the projection exposureapparatus, even in a state in which chamber 102 is filled with an inertgas like dried nitrogen and isolated from the atmosphere. Consequently,when performing the photolithography process to manufacture asemiconductor device, Step 1 is executed wherein the transmittance ofthe optical system part of projection exposure apparatus 100 (i.e.,reflective mirror 42 of the illumination optical system, condenser lens19, projection lens 21 and the like) is verified by the transmittancemeasurement system before starting up optical exposure apparatus 5, forexample (see FIG. 1), and proceeding to the exposure operation.Furthermore, the phenomenon wherein the transmittance of transmissiveoptical elements like condenser lens 19 and projection lens 21 dropsappears if the phenomenon wherein the reflectance of reflective opticalelements, such as reflective mirror 18 and the like, is reduced.

First, in Step 1, if projection exposure apparatus 100 transitions to astart-up state via a power supply (not shown) and light from lightsources 10 is from an ArF excimer laser, and reticle 20 is not present,control system 27 sets second detector 24, provided at one end ofworkpiece stage 23, in the exposure plane of projection lens 21 viadrive system 109.

Next, control system 27 discriminates, based on the output from firstdetector 44 b provided below reflective mirror 42 together with theoutput from second detector 24 provide at one end of workpiece stage 23,whether the light transmittance of a predetermined optical system partof projection exposure apparatus 100 has reached a predetermined value.In other words, the divider unit inside control system 27 calculates theratio of both outputs based on the output from first detector 4 and theoutput from second detector 12. Subsequently, the discrimination unitinside control system 27 discriminates, based on the calculation resultsthereof, whether the output from the divider unit has reached apredetermined threshold value. If the output from the divider unit hasnot reached a predetermined threshold value, operation transitions toStep 3 as the exposure operation discussed later, by means of thisdiscrimination unit. If the output from the divider unit has reached thepredetermined threshold value, operation proceeds to the next Step 2 asthe photo-cleaning process, by means of this discrimination unit.

Furthermore, in Step 1, it is preferable that control system 27discriminates, based on the output from second detector 24 provided atone end of workpiece stage 23, together with the output from thirddetector 112 provided at one end of reticle stage 107, whether the lighttransmittance of projection lens 21 of projection exposure apparatus 100has reached a predetermined value, and judges whether to proceed to Step2 after further including the discrimination results related to thelight transmittance of projection lens 21. At this point, in order toobtain the output from third detector 112, it is necessary that controlsystem 27 set third detector 112, provided at one end of reticle stage107, in the plane wherein the reticle should preferably be arranged.

In Step 2, to execute the photo-cleaning process, control system 27first sets the aperture size of variable aperture stop 14 via drivesystem 146. Accordingly, control system 27 rotates turret plate 140 viadrive system 146, and sets an appropriate aperture stop, 142 a or 142 h,in the illumination optical path so that the a value is 1 or greater.

To paraphrase, control system 27 rotates turret plate 140 via drivesystem 146 and sets an appropriate aperture stop, 142 a or 142 h, in theillumination optical path so that the following relation is satisfied:

NA_(I)≧NA_(P).

Consequently, an illumination light beam is sufficiently guided as faras the effective diameter of the optical elements like lenses comprisingcondenser lens 19 of the illumination optical system, the effectivediameter of the optical elements like lenses comprising projection lens21 and, further, the part beyond the effective diameters of theseoptical elements. As a result, moisture and organic matter and the likeadhering to the surfaces of these optical elements can be eliminated bythe photo-cleaning effect due to the exposure illumination light beam.

In addition, there is a possibility that the moisture and organic matterand the like that separated from the surfaces of optical elements bymeans of the photo-cleaning effect due to the irradiation of theexposure illumination light beam may be suspended in chamber 102 or in apredetermined plurality of spaces formed between a plurality of opticalelements. Consequently, control system 27 operates gas supply apparatus120 together with gas discharge apparatus 130 to forcibly discharge tooutside the apparatus moisture and organic matter and the like thatseparated from the surfaces of optical elements. In other words, basedon a command from control system 27, gas supply apparatus 120 suppliesinert gas like newly dried nitrogen into chamber 102 through pipe 122and inside projection lens 21 through pipe 124, respectively.Simultaneous therewith, gas discharge apparatus 130, based on a commandfrom control system 27, discharges to outside of projection exposureapparatus 100 the inert gas that includes moisture and organic matterand the like inside chamber 102 through pipe 132, and the inert gas thatincludes moisture and organic matter and the like inside projection lens21 through pipe 134.

Based thereon, since chamber 102 and projection lens 21 are filled withinert gas like nitrogen that has been cleaned by eliminating moistureand organic matter and the like, the transmittance of the optical systempart of the projection exposure apparatus can be restored to itsoriginal state. Furthermore, although inert gas like newly driednitrogen is supplied respectively into chamber 102 as well as projectionlens 21 by gas supply apparatus 120 during the photo-cleaning process ofStep 2, it is preferable to adopt a configuration wherein a strongoxidizing gas like oxygen (O₂), ozone (O₃) or active oxygen (O*) ismixed into the inert gas like nitrogen to be supplied. As a result, thephoto-cleaning effect is accelerated by the action of the strongoxidizing gas, and a larger effect can be expected. Also, at the sametime as the transition from the exposure process of Step 1 to thephoto-cleaning process of Step 2, air is flowed into at least one ofchamber 102 or projection lens 21. Subsequently, the photo-cleaningprocess of Step 2 can be started in an air environment, and can begradually replaced by the inert gas.

When the photo-cleaning process of Step 2 is completed, operationtransitions to the exposure operation as the third step. In Step 3, theactual exposure operation is executed. First, when reticle 20 is set onreticle stage 107 (namely the pattern surface of reticle 20 is set inthe object plane of projection lens 21), control system 27 sets theexposure surface 22 s of workpiece 22, supported by workpiece stage 23via drive system 109, in the image plane of projection lens 21.Simultaneous therewith, control system 27 sets the aperture size ofvariable aperture 14 via drive system 146.

Accordingly, the exposure conditions, like the σ value, and the exposuremap, in accordance with which workpiece 22 to be exposed is sequentiallycarried with each exposure completion, are pre-input into the memoryunit (not shown) inside control system 27 via input system 160, whichmay be a console. Based on this input information, control system 27rotates turret plate 140 via drive system 146, and sets one desiredaperture stop from among six aperture stops 142 b to 142 g for exposure.Based thereon, the pattern of reticle 20 under the condition of thedesired a value can be transferred onto workpiece 22 in a state whereinthe transmittance of a predetermined optical system of the projectionexposure apparatus is restored. As a result, a satisfactory microscopicpattern image can be faithfully transferred onto workpiece 22 and highlyintegrated satisfactory semiconductor devices can be manufactured withhigh throughput.

Furthermore, even if the exposure operation of the above Step 3 isexecuted, there are cases where contaminants like moisture and organicmatter adhere inside chamber 102 and projection lens 21, reducing thetransmittance of the optical system in the projection exposure apparatus100.

Consequently, if n is given as an integer 1 or greater, the exposureoperation is first stopped, operation returns to Step 1 and thetransmittance is measured after the exposure is completed of the n^(th)photosensitive substrate since the start of exposure and before theexposure of the n+1^(th) photosensitive substrate is executed. Forexample, operation returns to the transmittance measurement process ofStep 1 in a periodic step every time the exposure of 300-500 workpiece(wafers) 22 is completed. Then, control system 27 again confirms thetransmittance of a predetermined optical system part of projectionexposure apparatus 100 (reflective mirror 18 of the illumination opticalsystem, condenser lens 19 and projection lens 21 and the like) by meansof the transmittance measurement system.

If a drop in the transmittance of a predetermined optical system part ofthe projection exposure apparatus is not confirmed, operation returnsagain to Step 3 and the exposure operation continues. If a drop in thetransmittance of a predetermined optical system part of the projectionexposure apparatus is confirmed, operation returns again to Step 2 andthe photo-cleaning process is executed.

As can be understood from the above, since the illumination light beamis guided as far as a part beyond the effective diameter of each opticalelement of the optical system during exposure, it is preferable tochange the numerical aperture of the illumination optical system so thatthe condition NA_(I1)>NA_(I2) in the present example is satisfied,wherein NA_(I1) is the numerical aperture of the illumination opticalsystem in the photo-cleaning process of the abovementioned Step 2, andNA_(I2) is the numerical aperture of the illumination optical system inthe exposure process of the abovementioned Step 2.

Furthermore, since the illumination light beam is more reliably andsufficiently guided as far as the part beyond the effective diameter ofeach optical element of the optical system during exposure, it ispreferable to change the numerical aperture of the illumination opticalsystem so that the condition NA_(I1)≧NA_(P) (in other words, thecondition of σ≧1) is satisfied.

Furthermore, in the example discussed above, the fact that the σ valuein the photo-cleaning process is set to 1 or above was mentioned.However, the present invention is not limited thereto, and the numericalaperture of the illumination optical system in the photo-cleaningprocess may be set so that the maximum σ value is larger than themaximum σ value during the exposure operation. In this case, in order toobtain a more satisfactory photo-cleaning effect, it is preferable tosatisfy the following relation:

NA _(I)≧0.85NA _(P)

or

NA _(I1)≧0.85NA _(P)

In addition, in the example discussed above, if photo-cleaning isperformed in the photo-cleaning process using a reticle forphoto-cleaning having a predetermined pattern like a diffractivegrating, a greater photo-cleaning effect can be expected, since thelight due to the diffracted light of the reticle for photo-cleaning andthe like can be guided throughout the optical system.

The above working example discussed the case wherein projection lens 21completely comprised refractive optical elements. However, the presentinvention is not limited thereto, and may comprise a catadioptric-typeprojection lens that includes refractive-type optical elements likelenses and reflective-type optical elements like mirrors. Furthermore,projection lens 21 may also mostly or completely comprisereflective-type optical elements like mirrors. If projection lens 21comprises mainly reflective-type optical elements like mirrors, themeasurement of the reflectance of the optical system predominates overthe transmittance of the optical system, but is still included in theconcept of the present invention wherein the transmittance of theoptical system is measured. Furthermore, since the wavelength of thelight recognized as necessary for photo-cleaning is a short wavelengthunder 200 nm, it is extremely effective to provide a photo-cleaningfunction in an optical exposure apparatus that exposes with a lightsource having a short wavelength under 200 nm.

Next, an example related to a method for manufacturing a projection lensfor a projection exposure apparatus will be explained.

With reference to FIG. 14, in Step 1, lens elements L₁-L₅, as theoptical elements comprising the projection lens in accordance withdesign values (e.g., predetermined lens data), and lens barrels B₁-B₅that support the lens elements, are manufactured. Specifically, lenselements L₁-L₅ are first fabricated using well-known lens fabricationmachines so that they have a radius of curvature and on-axis thicknessin accordance with the predetermined design values respectively based onpredetermined optical materials. Subsequently, an antireflection film isformed on the surfaces of the fabricated lens elements L₁-L₅ in order toefficiently transmit light of the exposure wavelength (hereafter, lightof 193 nm) by means like well-known vapor deposition apparatuses. Lensbarrels B₁-B₅ that support the lens elements are fabricated usingwell-known metalworking machines to a shape having predetermineddimensions respectively based on the predetermined materials (stainlesssteel and brass and the like). In addition, through-holes for injectingand discharging inert gas like nitrogen are fabricated in predeterminedlens barrels B₁-B₅, by means of metalworking machines.

When the manufacture of parts comprising projection lens 21 iscompleted, as described above, operation transitions to the projectionlens 21 assembly process in Step 2. In Step 2, Lens elements L₁-L₅manufactured in Step 1 are assembled in lens barrels B₁-B₅ manufacturedin the same Step 1, as shown in FIG. 14, and five divided lens barrelunits are manufactured. Furthermore, at this point, injection pipes 124a -124 d, to which injection side valves V₁₁-V₁₄ are attached, anddischarge pipes 134 a -134 d, to which discharge side valves V₂₁-V₂₄ areattached, are attached to numerous through holes formed in predeterminedfive lens barrels B₁-B₅.

As shown in FIG. 14, when the manufacture of the above five divided lensbarrel units is completed, assembly of the divided lens barrel units iscompleted by ordering and adjusting each divided lens barrel unit whileinterposing washers WA₁-WA₄. Then, after the projection lens isassembled, in order to confirm the optical performance thereof, a testpattern (not shown) is arranged at the object plane (not shown) ofprojection lens 21, for example, and the test pattern image (not shown)formed at the image plane (not shown) of projection lens 21 is observedthrough a television camera (not shown). When the assembly process ofprojection lens 21 in the above Step 2 is completed, operationtransitions to the photo-cleaning process.

The work in the above Step 1 and Step 2 must basically be performed inthe atmosphere (air). In such an environment, moisture and organicmatter contained in the atmosphere adheres to the surfaces of lenselements L₁-L₅, resulting in greatly decreasing the transmittance of theassembled projection lens 21.

Accordingly, in Step 3, projection lens 21 assembled in Step 2 isirradiated with light of a wavelength the same as the exposure light,and the moisture and organic matter adhering to the surfaces of the lenselements comprising projection lens 21 are eliminated. Furthermore,since the wavelength of light recognized as necessary to performphoto-cleaning is a short wavelength under 200 nm, it is effective touse the photo-cleaning process when manufacturing a projection lens foran optical exposure apparatus that exposes with light of a shortwavelength under 200 nm.

FIG. 15 shows a schematic block diagram of photo-cleaning apparatus 170for irradiating projection lens 21 assembled in Step 2 with light of awavelength the same as the exposure light. The components comprisingapparatus 170 having the same function as in projection exposureapparatus 100 of FIG. 11 are assigned the same symbols. With referenceto FIG. 15, projection lens 21 assembled in Step 2 is first mountedinside chamber 102 of the photo-cleaning apparatus. In other words,projection lens 21 is mounted so that object plane OP of projection lens21 coincides with surface to be irradiated at the rear focal position ofcondenser lens 19 in the illumination optical system. Next, fourinjection pipes 124 a-124 d, to which injection valves V₁₁-V₁₄ areattached, as shown in FIG. 14, are connected to injection pipe 124 shownin FIG. 15. In addition, four discharge pipes 134 a-134 d to whichdischarge valves V₂₁-V₂₄ are attached, are connected to discharge pipe134 shown in FIG. 15. When the above setup is completed, as shown inFIG. 15, control system 27 operates gas supply apparatus 120 and gasdischarge apparatus 130. Inert gas like dried nitrogen is respectivelysupplied into chamber 102 and projection lens 21 by means of gas supplyapparatus 120. The air inside chamber 102 and projection lens 21 isdischarged by means of gas discharge apparatus 130. Then, the inert gasinside chamber 102 and projection lens 21 is replaced with air.

When chamber 102 and projection lens 21 are sufficiently filled withinert gas, ArF excimer laser light from light source 10, that oscillatespulsed light having an output wavelength equal to the exposurewavelength (for example, 193 nm) is guided to the photo-cleaningapparatus 170 through window 104. The laser light passing through lighttransmitting window 104 is reflected by reflective mirror 174, guided tofly's-eye lens 13 (i.e., optical integrator 13) and forms numerous lightsources (secondary light sources) on the exit surface side of fly's-eyelens 13.

Aperture stop 14 having a circular aperture of a predetermined size isprovided at the position wherein the numerous secondary light sourcesformed by this fly's-eye lens 13 are formed. Aperture stop 14 isconstituted so that it satisfies the condition NA_(I1)≧NA_(P) (namely,the condition of σ≧1), wherein NA_(I1) is the numerical aperture of theillumination optical system in the present process (photo-cleaningprocess).

After the light beam from the numerous secondary light sources isreflected by reflective mirror 18, it is condensed by condenser lens 19and superimposingly and evenly illuminates the surface to be irradiated.The light beam that passed through this surface to be irradiated passesthrough projection lens 21. By means thereof, moisture and organicmatter and the like adhering to the surface of the plurality of opticalelements L₁-L₅ comprising projection lens 21 can be eliminated by thephoto-cleaning effect due to the exposure illumination light beam.

Due to the photo-cleaning effect owing to the irradiation of the aboveexposure illumination light beam, there is a possibility that moistureand organic matter and the like that separated from the surfaces of theplurality of optical elements L₁-L₅ comprising projection lens 21 may besuspended in chamber 102 or in the predetermined plurality of spacesformed between the plurality of optical elements. Consequently, controlsystem 27 operates, simultaneous with the execution of thephoto-cleaning process, gas supply apparatus 120 and gas dischargeapparatus 130 to forcibly discharge to outside the apparatus themoisture and organic matter and the like that separated from thesurfaces of the optical elements. In other words, based on a commandfrom control system 27, gas supply apparatus 120 supplies inert gas likenewly dried nitrogen into chamber 102 through pipe 122 and intoprojection lens 21 through pipe 124, respectively. Simultaneoustherewith, based on a command from control system 27, gas dischargeapparatus 130 discharges to outside the projection exposure apparatusthe inert gas that includes the moisture and organic matter and the likeinside chamber 102 through pipe 132, and the inert gas that includes themoisture and organic matter and the like inside projection lens 21through pipe 134.

As a result, since chamber 102 and projection lens 21 are filled withinert gas like nitrogen that has been cleaned by eliminating moistureand organic matter and the like, it is possible to restore thetransmittance of projection lens 21 to its original condition.

Furthermore, when performing the photo-cleaning process of Step 3, inertgas like newly dried nitrogen is supplied by gas supply apparatus 120into chamber 102 and projection lens 21, respectively. However, it ispreferable to mix a strong oxidizing gas like oxygen (O₂), ozone (O₃) oractive oxygen (O*) into the inert gas like the nitrogen supplied by gassupply apparatus 120. As a result, the photo-cleaning effect isaccelerated due to the action of the strongly oxidizing gas, and agreater effect can be expected.

In addition, it is preferable to measure the transmittance of projectionlens 21 before and after executing the photo-cleaning process of Step 3.In this case, moveable third detector 112 is arranged along the surfaceto be irradiated of the illumination optical system, and moveable seconddetector 24 is arranged along the image plane of projection lens 21.When measuring the transmittance of projection lens 21, third detector112 is set in the surface to be irradiated, and second detector 24 isset in the image plane of projection lens 21.

Then, control system 27 may be constituted so that, based on the outputfrom third detector 112 and the output from second detector 24, itdiscriminates whether the light transmittance of projection lens 21 hasreached a predetermined value. Consequently, if the light transmittanceof projection lens 21 is measured before executing the photo-cleaningprocess of Step 3, how long the ArF excimer laser light from lightsource 10 should be irradiated can be estimated. In addition, if thetransmittance of projection lens 21 is measured after executing thephoto-cleaning process of Step 3, the restoration status of the lighttransmittance of projection lens 21 can be confirmed.

If it is necessary to readjust projection lens 21 at the stage where thephoto-cleaning process of the above Step 3 has completed, projectionlens 21 is removed from chamber 102 of projection exposure apparatus 170in FIG. 15 and the operation returns to Step 2. In addition, if it isconfirmed at the completion of Step 3 that a sufficient image formationperformance for projection lens 21 was obtained in the abovementionedStep 1, the projection exposure apparatus is completed by removing theprojection lens from chamber 104 of the photo-cleaning apparatus in FIG.5, and attaching it to the optical exposure apparatus body as shown inFIG. 1.

With respect to the embodiment as shown in FIG. 11 and FIG. 15, whenprojection lens 21 comprises a plurality of optical mirrors (e.g., planemirror, concave mirror or convex mirror), it is possible that thecontrol system 27 detects a reflectance of the projection lens 21 basedupon output signals from second and third detectors 24 and 112. Furtherwith respect to the embodiment as shown in FIG. 11 and FIG. 15, when theillumination optical system comprises a plurality of optical mirrors(e.g., plane mirror, concave mirror or convex mirror), it is possiblethat the control system 27 detects a reflectance of the illuminationoptical system based upon the output signal from second detector 24.

With reference now to FIG. 16, an optical exposure apparatus 200 isdescribed, which includes many of the same components included inoptical exposure apparatus 50 of FIG. 3. Optical exposure apparatus 200comprises, along optical axis A, a light source 10 emitting a light beam11 and a beam matching unit 204 that connects the light source to anillumination optical system 208. Light beam 11 impinges on illuminationoptical system 208 through beam matching unit 204 and is bifurcated intotwo optical paths, 11 a and 11 b, by beam splitter 42 provided withinillumination optical system 208. Beam splitter 42 is shown provided inthe middle of optical illumination system 208, but the location of thebeam splitter is not so limited. For example, beam splitter 42 may bearranged between illumination optical system 208 and optical member 212,which is described in greater detail below. Light beam 11 a, which istransmitted through beam splitter 42, is used as the illumination light(i.e., forms an illumination light beam). On the other hand, light beam11 b, which is reflected by beam splitter 42, impinges on detector 44 b.

Adjacent illumination optical system 208 is a photo-cleaning opticalmember 212 removably arranged at object plane OP, where a mask (notshown) is typically arranged. Photo-cleaning optical member 212, alongwith another photo-cleaning optical member 216, are supported by asupport apparatus 220. Either one of photo-cleaning optical members 212and 216 can be arranged at object plane OP of optical exposure apparatus200 by rotationally driving support apparatus 220 by drive system 32.The mask (not shown) is arranged at object plane OP when the normalexposure process is performed. Both the mask and photo-cleaning opticalmembers 212 and 216 are supported by support apparatus 220.

With continuing reference to FIG. 16, a light beam 222 emanates fromillumination optical system 208 and is refracted by one of opticalmembers 212 and 216. The direction of refraction depends on whichphoto-cleaning optical member is inserted in the optical path defined bylight beam 222. Upon refraction, light beam 222 becomes light beam 223,which passes through projection lens 21 and then impinges on detector 24arranged on workpiece stage 23. A light detection system 26 calculatesthe transmittance of optical exposure apparatus 200 from beam splitter42 to detector 24 based on signals from detector 44 b and detector 24.The adhesion state of contaminants on the lens surfaces (not shown)comprising projection lens 21 is determined by comparing the calculatedtransmittance with the design transmittance. The timing of ceasingphoto-cleaning is determined by observing the progress of photo-cleaningthrough monitoring the change in transmittance. A control system 27 isconnected to light source 10, drive system 32, and a storage unit 230,the latter of which stores data related to exposing the workpiece (notshown). The amount of exposure light is controlled via control system 27based on the data in storage unit 230, as discussed later in moredetail.

With reference now to FIG. 17, the elements 13-19 comprisingillumination optical system 208 (i.e., the broken-line box) of opticalexposure apparatus 200 are the same elements 13-19 as described above inconnection with optical exposure apparatus 5 of FIG. 1. Light beam 222emanating from second objective lens 19 irradiates photo-cleaningoptical member 212, for example, arranged at object plane OP.

The function of photo-cleaning optical members 212 and 216 is nowexplained. To facilitate this explanation, photo-cleaning will beexplained for the case where no photo-cleaning optical member is used.With reference now to FIG. 18, light beam 222 from illumination opticalsystem 208 is “upstream” of object plane OP and light beam 223 is“downstream” of object plane OP. The illumination optical system side ofobject plane OP is the “upper part” U or “upstream” region, and the“lower part” L is the projection lens side or “downstream” region. Lightbeam 222 comprises light beams 222 a-222 c, which include principallight rays 233, 234, and 235, respectively. Principal light rays 233,234 and 235 pass through the left-most, center (i.e, axial) andright-most side of the drawing within the exposure range on object planeOP (i.e., within the outline of mask 20). In addition, rays 233 a and233 b of light beam 222 a impinge object plane OP at an angle α withrespect to principal ray 233. Likewise, rays 234 a and 234 b of lightbeam 222 b impinge object plane OP at an angle α with respect toprincipal ray 234, and rays 235 a and 235 b of light beam 222 c impingeat angle a with respect to principal ray 235. Here, the angle α isrelated to the illumination optical system 208 numerical aperture by therelation NA_(I)=n·sin α, where n in the present case is the refractiveindex of air (n=1).

With continuing reference to FIG. 18, if mask 20 is arranged at objectplane OP, light beams 222 a-222 c are diffracted by the mask. A portionof the diffracted light, as depicted by light rays 233 c, 233 d, 234 c,234 d, 235 c and 235 d, is generated wherein the angle β of these rays,as measured from their corresponding principal rays, is larger thanangle α. Angle β is the half-angle associated with the numericalaperture NA_(P) of projection lens 21 on the object (mask 20) side. Withreference now to FIG. 19, among the diffracted light rays generated bymask 20, only the light rays having an angle with respect to opticalaxis A less than angle β can pass through projection lens 21 and form animage on workpiece 22 arranged on workpiece stage 23.

With continuing reference to FIGS. 18 and 19, if no mask 20 is presentat object plane OP, light beam 222 on upper side U (i.e., on theprojection lens side) has an angle a the same as on lower side L (i.e.,on the illumination optical system side), as shown by the solid linerays. Consequently, if photo-cleaning of the lens surfaces comprisingprojection lens 21 is performed with the exposure light without mask 20present in object plane OP, then the exposure light does not irradiatethe lens surfaces that include angular regions 240 (shaded).Consequently, the portions of the lenses corresponding to angularregions 240 are not photo-cleaned.

With reference now to FIG. 20, to solve this shortcoming, a methodinvolving diffusing the illumination light (i.e., light beam 222) by adiffusion plate, such as discussed above in connection with diffusionplate 72 of apparatus 70 of FIG. 7, and proposed in Japanese PatentApplication No. Hei 9-155856, is one possible option. If diffusion plate72 is used, diffused light rays 251 having an angle greater than angle αis obtained, and only those diffused light rays having an angle lessthan angle β pass through projection lens system 21. As a result, thelens surfaces in the region greater than angle α and less than angle βcan also be photo-cleaned.

In contrast, the present invention is constituted so that the lenssurfaces of projection lens 21 in the region greater than angle α andless than angle β are photo-cleaned by refracting light beam 222 by aphoto-cleaning optical member (for example, a convex lens or concavelens and the like) having refractive power.

With reference now to FIG. 21, a negative lens (hereinafter referred toas a concave lens) 216 with a principle plane 216P and having negativerefractive power is used as a photo-cleaning optical member.Photo-cleaning is achieved refracting each light beam 222 a-222 c inlight beam 222, with photo-cleaning concave lens 216. The latter isarranged such that principal plane 216P coincides with object plane OP,and the central axis (not shown) of the concave lens coincides withoptical axis A. Principle rays 233 and 235 are refracted by concave lens216 such that they become more distant from optical axis A. Therefractive power (focal length) of concave lens 216 is set such thatrays 233 b and 235 a, which form an angle α with respect to optical axisA before refraction, form an angle equal to angle β after refraction.The method for setting the focal length of concave lens 216 is discussedin further detail below.

With reference now to FIG. 22, a positive lens (hereinafter referred toas a convex lens) 212 with a principle 212P and having positiverefractive power is used as a photo-cleaning optical member.Photo-cleaning is achieved by refracting each light beam 222 a-222 c inlight beam 222 with photo-cleaning convex lens 212. The latter isarranged such that principal plane 212P coincides with object plane OPand the central axis (not shown) of the convex lens coincides withoptical axis A. Principle rays 233 and 235 are refracted by convex lens212 such that they approach optical axis A. The refractive power (focallength) of convex lens 212 is set such that the angles of rays 233 a and235 b with respect to optical axis A become, after refraction, nearlyequal to angle β. Furthermore, the angular change Θ of rays 233 a and235 b before and after refraction is expressed as:

Θ=β−α=sin⁻¹(NA _(P))−sin⁻¹(NA _(I))

Next, with reference to FIGS. 23a and 23 b, the method of setting focallength f of concave lens 216 and convex lens 212 is explained. FIG. 23ashows the case for concave lens 216, and 23 b shows the case for convexlens 212. Principal rays 233 and 235 refract at nearly angle Θ ifangular change Θ arises as with rays 233 b-235 a in FIG. 21 and rays 233a-235 b in FIG. 22. Thus, the following sets the focal length with theangle of refraction as Θ. In FIGS. 23a and 23 b, F is the focal point ofthe lens in question. For concave lens 216, F₂₁₆ is the point ofintersection when principal rays 233 and 235 after refraction, areextended toward upper side U (i.e., the upper portion of FIG. 23a). Forconvex lens 212, F₂₁₂ is the point where principal rays 233 and 235,after refraction, intersect on lower side L (i.e., the lower portion ofthe FIG. 23b). Focal length f₀ is given by the equation

|f ₀ |=d/tan Θ=d/[tan{sin⁻¹(NA _(P))}−tan{sin⁻¹(NA _(I))}]

wherein d is the distance of principal rays 233 and 235 from opticalaxis A, as measured at object plane OP. In the case of concave lens 212,which is a negative lens, focal length f₀ is less than 0.

The magnitude |f| of focal length f of the photo-cleaning optical membershould be set to less than |f₀|, as defined in the above equation.

FIGS. 24a and 24 b show light beams 222 a and 222 c (i.e., the lightbeams associated with principal rays 233 and 235) refracted by concavelens 216 (FIG. 24a) and by convex lens 212 (FIG. 24b), with f=f₀, toform light beams 223 a and 223 c. The latter then pass throughprojection lens 21, and form an image on photosensitive workpiece 22. InFIGS. 16, 17, and 19, projection lens 21 is shown as a single lens, butone skilled in the art will appreciated that the projection lenstypically comprises a plurality of lenses.

With continuing reference to FIGS. 24a and 24 b, the case where lensesL1 and L2 comprising projection lens 21 and provided at the positionsshown therein is now considered. Regions R1 and R2 in lens L2 shown inFIG. 24a are regions through which the exposure light passes duringexposure of the mask (not shown). However, it can be seen that regionsR1 and R2 are not irradiated by the exposure light if concave lens 216is used as the photo-cleaning optical member. Consequently, if concavelens 216 is used as the photo-cleaning optical member, regions R1 and R2on the lens surfaces of lens L2 cannot be photo-cleaned.

On the other hand, with reference to FIG. 24b, if convex lens 212 isused as the photo-cleaning optical member, the regions R3 and R4 on thesurfaces of lens LI are not irradiated by the photo-cleaning exposurelight. Accordingly, the lens surfaces at regions R3 and R4 cannot bephoto-cleaned.

With reference now to both FIGS. 24a and 24 b, it can be seen that theregions R1 and R2 of lens L2 can be photo-cleaned if convex lens 212 isused, and the regions R3 and R4 of lens L1 can be photo-cleaned ifconcave lens 212 is used. In this manner, there is a risk that a lenssurface will arise that cannot be photo-cleaned by the exposure lightwith just one of concave lens 216 and convex lens 212 by virtue of thelens configuration of projection lens 21. Accordingly, it is preferableto perform photo-cleaning alternately using both concave lens 216 andconvex lens 212.

With reference again to FIG. 17, there is a possibility of contaminationof lens surfaces immediately after manufacturing the optical exposureapparatus (e.g., optical exposure apparatus 200), when replacing themask, if the optical exposure apparatus has not been used for a longperiod of time, or due to maintenance of the optical exposure apparatusand the like. In these cases, photo-cleaning is performed by arranging,before starting the exposure process, the abovementioned cleaningoptical members 216 and 212, at object plane OP, and performing thephoto-cleaning operation. At this point, the transmittance of projectionlens 21 is measured by detectors 44 b and 24. If the transmittance ofprojection lens 21 due to light irradiation from several minutes toseveral hours reaches a level that does not significantly change, thenthe photo-cleaning optical member (e.g., concave lens 216 or convex lens212) is withdrawn from object plane OP. A process mask is then arrangedat object plane OP, a photosensitive workpiece 22 is arranged onworkpiece stage 23, and the exposure process is started (see FIG. 19).In the mode for carrying out the present invention, as discussed above,one of photo-cleaning optical members 216 and 212 is provided at objectplane OP. However, the photo-cleaning elements need not be so arranged.For instance, they may be provided on the projection lens side ofaperture stop 16, which determines the numerical aperture ofillumination optical system 208. Aperture stop 16 may, in turn, belocated between projection lens 21 and light source 10.

Photo-cleaning Procedure

A specific method for photo-cleaning according to the present inventionis now explained. Generally, reductions in the transmittance ofprojection lens can be broadly classified as being either of two cases:

(1) a drop in the transmittance of the lens elements themselves, and

(2) the adherence of contaminants to the lens surfaces.

In case (1), the transmittance of the entire projection lens can drop asfar as approximately 80%. However, in this case, only a drop in theillumination intensity on the photosensitive substrate occurs, andillumination uniformity (i.e., evenness) does not deteriorate. In case(2) as well, the same reduction in overall illumination intensity as incase (1) arises if the extent of contamination is light. However, if theextent of contamination is heavy, then this is no longer true. Forexample, if air directly contacts the lenses immediately aftermanufacturing of the apparatus or due to maintenance of the opticalsystem and the like, then the transmittance can drop as far asapproximately 10% in the worst case, as compared to the case of a lenshaving no contamination at all. Further, deterioration of illuminationuniformity arises on the photosensitive substrate due to transmittancenon-uniformity.

As discussed above, if the contamination becomes heavy and theillumination uniformity deteriorates, it is necessary to performphoto-cleaning uniformly over the entire lens surface. On the otherhand, in case (1), or in case (2) with small amounts of contamination,it is possible to correct the change in transmittance by controlling theexposure light quantity, as discussed below. Furthermore, when measuringthe illumination uniformity, the exposure light quantity (i.e.,intensity) at each location in the exposure region (i.e., image field)is measured using detector 24 (see FIG. 16). The difference between themaximum value and the minimum value of the exposure light quantity atthat time is calculated, and the illumination uniformity is judged toassess whether it has deteriorated. Photo-cleaning is performed whenthis difference (i.e., the non-uniformity) exceeds a specified value.

Explanation of Exposure Based on Exposure Light Quantity Control

FIG. 25 shows a graph 280 representing a time-varying characteristic ofthe transmittance of projection lens 21 when laser light is irradiatedthereon and in which transmittance has uniformly dropped. Thetransmittance drops greatly immediately after the start of irradiationof laser light (“START EXPOSURE”), but rises gradually thereafter andnearly reaches the saturation state with the elapse of a certain amountof time, as explained in greater detail below. The extent of thetransmission drop immediately after the start of laser irradiationdepends on changes in the internal characteristics of the glasscomprising the lenses. The phenomenon of gradual recovery thereafter isdue to the removal, by laser light irradiation, of contaminants (waterand organic substances) adhering to the lens surfaces.

In the present mode for carrying out the photo-cleaning method of thepresent invention, the time varying transmittance characteristicrepresented by graph 280 is pre-stored in storage unit 230 (FIG. 16).Occasionally, the exposure operation is interrupted due to, for example,the replacement operation of a photosensitive substrate (e.g., workpiece22), or if optical exposure apparatus 200 is stopped for a long periodof time (e.g., on the order of ten hours). In this case, thetransmittance of projection lens 21 is first measured using detectors 44b and 24 prior to restarting the exposure operation. With continuingreference to FIG. 25 and graph 280, if the transmittance prior torestarting exposure is given as T_(A), it is assumed that thetransmittance changes as time proceeds in the right direction along thehorizontal time axis from t₀ when the exposure is restarted (startingtime t₀). Furthermore, the exposure light quantity from light source 10is adjusted, based on the time-varying transmittance characteristicstored in storage unit 230, such that the illumination intensity on thephotosensitive substrate is maintained at the appropriate level.Thereafter, operation shifts to normal exposure without controlling theexposure light quantity if the change in transmittance becomes small(i.e., the time lapsed is t₁, and the transmittance is T_(B)).

With reference now to FIG. 26, flowchart 300 outlines the procedure forexposure and controlling the exposure light quantity, discussed above,and for photo-cleaning after manufacturing the optical exposureapparatus, after maintenance, or other interruptions. Flow chart 300also shows the procedure from the “start” (Step S0) to the “end” (StepS6) of the exposure operation.

First, step S1 determines whether the optical exposure apparatus is in astate immediately after assembly (i.e., immediately after it ismanufactured), or after maintenance. If immediately after assembly ormaintenance, the flow proceeds to step 2, photo-cleaning, as discussedin greater detail below. Otherwise, it proceeds to step S11, alsodiscussed below. Assuming having proceeded to photo-cleaning step S2,the flow proceeds next to step S3 in which normal exposure is performedwithout exposure light quantity control. Step S4 then inquires whetherthe exposure is interrupted. If the answer to this query is YES, thenthe flow proceeds to step S11. If not interrupted, the flow proceeds tostep S5, which inquires whether the exposure has ended. If the query instep S5 is answered YES, the exposure procedure ends at step S6. If thequery in step S5 is answered NO, the flow returns to step S3 and normalexposure is performed.

On the other hand, if the flow proceeds from step S1 to step S11, thenstep S11 inquires whether the transmittance is lower than T_(B) of graph280, i.e., whether the exposure light quantity corresponds to thenecessary transmittance. If the answer to this query is YES, the flowproceeds to step S12. If the answer to this query is NO, then the flowproceeds to step S3, and normal exposure is performed. Step S12 inquireswhether the illumination uniformity on the photosensitive substrate(i.e., workpiece 22) has exceeded a specified (predetermined) value anddeteriorated. If so, the flow proceeds to step S2, and photo-cleaning isperformed. If there is no deterioration in the illumination uniformity,the flow proceeds to step S13, and exposure is performed whilecontrolling the exposure light quantity. Step S14 inquires whether theexposure time has reached or exceeded a predetermined time (t₁-t₀),namely whether the change in transmittance has become sufficiently smalland greater than or equal to T_(B). If the answer to this query is YES,the flow proceeds to step S3, and exposure is performed under normalexposure starting from the next exposure. On the other hand, if theanswer to this query is NO, the flow returns to step S11.

With reference now to FIG. 27, a specific procedure for photo-cleaningperformed in step S2 of flow chart 280 is explained. Process PR1 purgesN2 gas (nitrogen gas) through projection lens 21, i.e., between thelenses in the lens barrels therein, immediately after manufacturing theoptical exposure apparatus or after maintenance thereof. In the nextprocess PR2, if concave lens photo-cleaning optical member 216 (see FIG.16) is arranged at object plane OP in the optical path, photo-cleaningis performed by irradiating the lens surfaces of projection lens 21 withthe exposure light in process PR3. In this case, after arranging concavelens photo-cleaning optical member 216 at object plane OP, convex lensphoto-cleaning optical member 212 is arranged at the object plane andphoto-cleaning is performed by irradiating the lens surfaces ofprojection lens 21 with the exposure light. Next, in process PR4, thetransmittance of projection lens 21 is measured using detectors 44 b and24. Photo-cleaning is completed if the measured transmittance reaches orexceeds a predetermined value (e.g., T_(B) in graph 280). In addition,if the transmittance is smaller than predetermined value T_(B),photo-cleaning is performed until the transmittance reaches or exceedspredetermined value T_(B).

The present mode for carrying out the invention, as discussed above, canilluminate the entire numerical aperture NA_(P) region of projectionlens 21 by refracting the illumination light (i.e., exposure light),using photo-cleaning optical members concave lens 216 or convex lens212. As a result, the contaminants adhering to the lens surfaces withinthe numerical aperture NA_(P) region can be eliminated by photo-cleaningwith the exposure light. In addition, in the present mode for carryingout the invention, the entire numerical aperture NA_(P) region isilluminated by refracting light beam 222 (see FIG. 16). Thus, moreeffective photo-cleaning can be performed without any reduction in thequantity of light, such as occurs in the method wherein illuminationlight is diffused using a diffusion plate, as discussed earlier.

It will be understood by one skilled in the art that the above-describedembodiment of the invention using concave lens 216 and convex lens 212,is not so limited. For example, any optical member having a positive ornegative refractive power can be used. In addition, a reflecting opticalmember (e.g., a mirror) having reflecting power may be provided on theillumination optical system side as the photo-cleaning optical member.Furthermore, in the present mode for carrying out the invention, thephoto-cleaning effectiveness can be further increased by setting thenumerical aperture of the illumination optical system greater than orequal to the effective diameter of the optical elements comprisingprojection lens 21. In addition, photo-cleaning may also be performed byproviding a photo-cleaning light source separate from the exposure lightsource, and using the light of the photo-cleaning light source forphoto-cleaning. In this case, it is preferable to use a light sourcethat emits light of a wavelength nearly equal to the wavelength of theexposure light. In addition, in the mode for carrying out the presentinvention, an ArF laser was considered as a light source for generatingthe exposure light, for the sake of explanation. However, the presentinvention can also be applied to an optical exposure apparatus that usesextreme ultra-violet light (EUVL) like soft X-rays having a shortwavelength.

Also, in the present invention, support apparatus 220 and drive system32 constitute a lens-moving apparatus, light detection system (i.e.,transmittance measuring apparatus) 26 constitutes a transmittancecalculating means, control system 27 constitutes a controlling means,storage unit 230 constitutes a storage means, and detectors 44 b and 24,and light detection system 26 constitute a detection means.

To summarize, according to the present invention as described above,moisture and organic matter and the like adhering to optical elementsconstituting the projection lens can be eliminated by means ofphoto-cleaning. This allows for a satisfactory mask pattern image to betransferred onto a photosensitive substrate. Even if the transmittanceof the projection lens drops in a photolithography process that includesan exposure method, the transmittance of the projection lens can besufficiently restored by newly adopting a photo-cleaning process.Consequently, semiconductor devices like LSIs having a higher level ofintegration can be manufactured, since a finer pattern can betransferred onto a photosensitive substrate. In addition, if thephoto-cleaning process according to the present invention is used whenmanufacturing the projection lens, it becomes possible to performassembly and maintenance of the projection lens in a normal workingenvironment, and the transmittance of the projection lens can besufficiently ensured.

Further, as explained above, according to the present invention, even ifthe numerical aperture NA_(I) of the illumination optical system issmaller than the numerical aperture of the projection lens NA_(P), theentire surface of the lens surface region (numerical aperture region)corresponding to the numerical aperture of the projection lens can beilluminated by refracting the light beam from the illumination opticalsystem by a photo-cleaning optical member, without reducing the quantityof light of the light beam. As a result, contaminants adhering to thenumerical aperture region of the lens surface can be more effectivelyeliminated.

In particular, according to one embodiment of the present invention, thephoto-cleaning optical member comprises one of a positive lens and anegative lens, and the positive lens and negative lens can beselectively used in accordance with the lens configuration of theprojection lens. Consequently, the entire numerical aperture region ofthe lens surfaces of the projection lens can be photo-cleaned,regardless of the lens configuration of the projection lens. Inaddition, since the mask and photo-cleaning optical member(s) aresupported by the support apparatus and are selectively arranged at theobject plane, the time required for photo-cleaning can be shortened.Also, since exposure is performed while continuing to control (a)photo-cleaning or (b) the cumulative exposure light quantity inaccordance with the extent of illumination uniformity, high-precisionexposure can be performed.

The above-described embodiments are meant to clarify the technicaldetails of the present invention. For instance, the optical componentsare described and shown in the figures as single elements, whereas inpractice one skilled in the art will know that such components mayinclude multiple elements. In addition, the above-described embodimentscan be combined (e.g., the oscillating lens embodiment can be employedin optical exposure apparatus 5 with certain obvious modifications) tocreate other optical exposure apparatuses with photo-cleaningcapability. Thus, since certain changes may be made to the apparatusesand methods disclosed herein with respect to the to the aforementionedpreferred embodiments without departing from the scope of the invention,it is intended that all matter contained in the above description andshown in the accompanying drawings be interpreted in an illustrative andnot in a limiting sense.

What is claimed is:
 1. A method for manufacturing a projection opticalsystem, comprising the steps of: assembling the projection opticalsystem so as to form a pattern onto a photosensitive substrate; andrecovering from a decrease of light intensity caused by said projectionoptical system, wherein the step of recovering comprises separating atleast one contaminant that is adhered to at least one surface of atleast one optical element of the projection optical system from the atleast one surface and the step of recovering is performed at a time whenthe projection optical system is not being used to expose a workpiece tolight.
 2. A method according to claim 1, wherein the step of recoveringfrom a decrease of light intensity comprises the step of illuminatingsaid projection optical system with light having a wavelength less than200 nanometers.
 3. A method according to claim 1, further comprising thestep of supplying a predetermined gas to at least a portion of saidprojection optical system.
 4. A method according to claim 3, furthercomprising the step of discharging the predetermined gas from said atleast a portion of said projection optical system.
 5. A method accordingto claim 3, wherein the predetermined gas includes inert gas.
 6. Amethod according to claim 3, further comprising the step of measuringthe light intensity with respect to light passed through said projectionoptical system.
 7. A method according to claim 6, further comprising thestep of adjusting said projection optical system.
 8. A method accordingto claim 1, further comprising the step of measuring light intensitywith respect to light passed through said projection optical systembefore the step of recovering from a decrease of light intensity.
 9. Amethod according to claim 8, further comprising the step of adjustingsaid projection optical system.
 10. A method according to claim 1,further comprising the step of adjusting said projection optical systemafter the step of recovering from a decrease of light intensity.
 11. Amethod for manufacturing a projection optical system, comprising thesteps of: assembling said projection optical system so as to form apattern onto a photosensitive substrate; and eliminating a contaminationof at least one optical element of said projection optical system,wherein the step of eliminating comprises separating at least onecontaminant that is adhered to at least one surface of the at least oneoptical element of the projection optical system from the at least onesurface and the step of eliminating is performed at a time when theprojection optical system is not being used to expose a workpiece tolight.
 12. A method according to claim 11, wherein the step ofeliminating a contamination comprises the step of illuminating saidprojection optical system with light having a wavelength less than 200nanometers.
 13. A method according to claim 11, further comprising thestep of supplying a predetermined gas to at least a portion of saidprojection optical system.
 14. A method according to claim 13, furthercomprising the step of discharging the predetermined gas from said atleast a portion of said projection optical system.
 15. A methodaccording to claim 13, wherein the predetermined gas includes inert gas.16. A method according to claim 13, further comprising the step ofmeasuring the light intensity with respect to light passed through saidprojection optical system.
 17. A method according to claim 16, furthercomprising the step of adjusting said projection optical system.
 18. Amethod according to claim 11, further comprising the step of measuringlight intensity with respect to light passed through said projectionoptical system before the step of eliminating a contamination.
 19. Amethod according to claim 18, further comprising the step of adjustingsaid projection optical system.
 20. A method according to claim 11,further comprising the step of adjusting said projection optical systemafter the step of eliminating a contamination.
 21. A method formanufacturing an optical system, comprising the steps of: assemblingsaid optical system; supplying a predetermined gas to at least a portionof said optical system; and recovering from a decrease of lightintensity caused by said optical system, wherein the step of recoveringcomprises separating at least one contaminant that is adhered to atleast one surface of at least one optical element of the optical systemfrom the at least one surface and the step of recovering is performed ata time when the optical system is not being used to expose a workpieceto light.
 22. A method according to claim 21, further comprising thestep of adjusting said optical system.
 23. A method according to claim21, further comprising the step of measuring the light intensity withrespect to light passed through said optical system.
 24. A methodaccording to claim 21, wherein the predetermined gas includes inert gas.25. A method according to claim 21, further comprising the step ofdischarging the predetermined gas from said at least a portion of saidoptical system.
 26. A method for manufacturing an optical system,comprising the steps of: assembling said optical system; supplying apredetermined gas to at least a portion of said optical system; andeliminating a contamination of at least one optical elements of saidoptical system, wherein the step of eliminating comprises separating atleast one contaminant that is adhered to at least one surface of the atleast one optical element of the optical system from the at least oneoptical element and the step of eliminating is performed at a time whenthe optical system is not being used to expose a workpiece to light. 27.A method according to claim 26, further comprising the step of adjustingsaid optical system.
 28. A method according to claim 26, furthercomprising the step of measuring the light intensity with respect tolight passed through said optical system.
 29. A method according toclaim 26, wherein the predetermined gas includes inert gas.
 30. A methodaccording to claim 26, further comprising the step of discharging thepredetermined gas from said at least a portion of said optical system.31. A method for manufacturing a semiconductor device, comprising thesteps of: illuminating a reticle; and transferring a pattern of saidreticle onto a photosensitive substrate by using the projection opticalsystem manufactured by the method according to claim
 7. 32. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the projection optical systemmanufactured by the method according to claim
 9. 33. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the projection optical systemmanufactured by the method according to claim
 1. 34. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the projection optical systemmanufactured by the method according to claim
 17. 35. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the projection optical systemmanufactured by the method according to claim
 19. 36. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the projection optical systemmanufactured by the method according to claim
 11. 37. A method formanufacturing a semiconductor device, comprising the steps of:illuminating a reticle; and transferring a pattern of said reticle ontoa photosensitive substrate by using the optical system manufactured bythe method according to claim
 21. 38. A method for manufacturing asemiconductor device, comprising the steps of: illuminating a reticle;and transferring a pattern of said reticle onto a photosensitivesubstrate by using the optical system manufactured by the methodaccording to claim
 26. 39. A method for manufacturing a projectionoptical system according to claim 1, wherein the at least onecontaminant is separated from the at least one surface of the opticalelement by photo-cleaning the at least one surface of the at least oneoptical element.
 40. A method for manufacturing a projection opticalsystem according to claim 11, wherein the at least one contaminant isseparated from the at least one surface of the optical element byphoto-cleaning the at least one surface of the at least one opticalelement.
 41. A method for manufacturing an optical system according toclaim 21, wherein the at least one contaminant is separated from the atleast one surface of the optical element by photo-cleaning the at leastone surface of the at least one optical element.
 42. A method formanufacturing an optical system according to claim 26, wherein the atleast one contaminant is separated from the at least one surface of theoptical element by photo-cleaning the at least one surface of the atleast one optical element.